High Performance Bifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall Water Splitting

ABSTRACT

A method of manufacturing a bifunctional electrocatalyst for overall water splitting comprising oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) by growing electrocatalyst comprising primarily metallic phosphides on three-dimensional substrate by: immersing the substrate in an iron nitrate solution to form a once disposed substrate; subjecting the once disposed substrate to thermal phosphidation with phosphorus powder under inert gas to grow metal phosphides thereupon and form a once subjected substrate; cooling the once subjected substrate to form a cooled, once subjected substrate; immersing the cooled, once subjected substrate in an iron nitrate solution to form a twice disposed substrate; and subjecting the twice disposed substrate to thermal phosphidation with phosphorus powder under inert gas to provide an electrode comprising the bifunctional electrocatalyst on the three-dimensional substrate. An electrode for overall water splitting having a substrate and a bifunctional electrocatalyst comprising primarily metallic phosphides on a surface of the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/047,032 filed Oct. 12, 2020, and entitled “High PerformanceBifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall WaterSplitting,” which is a 35 U.S.C. § 371 national stage application ofPCT/US2019/026814 filed Apr. 10, 2019, and entitled “High PerformanceBifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall WaterSplitting,” which claims priority to U.S. Patent Application No.62/656,562, filed Apr. 12, 2018, and entitled “High PerformanceBifunctional Porous Non-Noble Metal Phosphide Catalyst for Overall WaterSplitting,” filed, the disclosures of which are incorporated herein byreference in their entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This research was sponsored in part by the US Department of Energy underContract No. DE-SC0010831.

TECHNICAL FIELD

The present disclosure relates to water splitting; more particularly,the present disclosure provides bifunctional electrocatalysts foroverall water splitting including hydrogen evolution reaction (HER) andoxygen evolution reaction (OER); still more particularly, the presentdisclosure provides a high performance, bifunctional, porous, non-noblemetal phosphide catalyst for overall water splitting.

BACKGROUND

The scalable storage of such abundant renewable energy sources as windor solar energy is sought to mitigate the aggravated global energycrisis while addressing the environmental issues. Converting solar- orwind-derived electricity to hydrogen fuel via water electrolysis (or‘water splitting’) is an appealing means to accomplish this energyconversion and storage technology. Water splitting is a term that refersto the chemical reaction where water is separated into its elements ofhydrogen and oxygen. This may be employed in order to obtain hydrogenfor various applications, including hydrogen fuel production. Hydrogen(H₂) production from electrochemical water splitting is a clean andsustainable energy resource that may be used to substitute fossil fuelsand meet rising global energy demand, since water is the only startingsource and byproduct during fuel burning in an engine.

There are two commercialized water electrolysis avenues, includingalkaline and proton exchange membrane (PEM) water electrolysis. PEMwater electrolysis has high energy efficiency with high hydrogenproduction rate, but requires noble metal (platinum (Pt) and iridium(Ir))-based catalysts, making it unfavorable due to high cost andscarcity. The alternative, low-cost alkaline water electrolysis, is amature technology for large-scale hydrogen production that is low-costdue to compatibility with non-noble catalysts, but it suffers from lowproduction rates. A challenge remains due to the huge energy penaltyincurred by the uphill reaction kinetics of the catalysts that requiresignificantly high cell voltages (1.8-2.4 V, far larger than thethermodynamic value of 1.23 V) to catalyze the reaction withelectrolysis currents of 200-400 mA/cm², resulting in the production ofless than 5% hydrogen by means of water electrolysis in the worldwideindustry.

Existing bifunctional catalysts to negotiate the overall water splittingefficiently in alkaline electrolytes can operate only steadily at lowcurrent density (e.g., less than 20 mA/cm²), not to mention the lowenergy conversion efficiency at above 200 mA/cm² required for commercialapplications. These catalysts are inadequate for industrial scale use,due to a difficulty in integrating both the merits of hydrogen evolutionreaction (HER) and oxygen evolution reaction (OER) electrocatalysts in asingle bifunctional catalyst in the same electrolyte (either alkaline oracid).

Accordingly, known bifunctional electrocatalysts fabricated for alkalinewater electrolysis do not simultaneously exhibit good performance forboth the HER and OER, especially the OER, and known bifunctional waterelectrolyzers do not provide high-current operation. Thus, there existsa need for a single bifunctional catalyst that provides outstanding HERand OER activities simultaneously in the same electrolyte. Desirably,such a bifunctional catalyst is a non-precious metal-based, robustbifunctional catalyst for promoting both cathodic hydrogen evolution andanodic oxygen evolution reactions, thus expediting overall watersplitting toward large-scale commercialization at high current densitieswith low cell voltages.

SUMMARY

Herein disclosed is a method of manufacturing a bifunctionalelectrocatalyst for overall water splitting comprising oxygen evolutionreaction (OER) and hydrogen evolution reaction (HER), the methodcomprising: growing electrocatalyst comprising primarily metallicphosphides on a three-dimensional substrate by: immersing the substratein an iron nitrate solution to form a once disposed substrate;subjecting the once disposed substrate to thermal phosphidation withphosphorus powder under inert gas to grow metal phosphides thereupon andform a once subjected substrate; cooling the once subjected substrate toform a cooled, once subjected substrate; immersing the cooled, oncesubjected substrate in an iron nitrate solution to form a twice disposedsubstrate; and subjecting the twice disposed substrate to thermalphosphidation with phosphorus powder under inert gas to provide anelectrode comprising the bifunctional electrocatalyst on thethree-dimensional substrate.

Also disclosed herein is an electrode for overall water splitting, theelectrode comprising: a substrate; and a bifunctional electrocatalystcomprising primarily metallic phosphides on a surface of the substrate.

Also disclosed herein is a method of electrocatalytic water splitting,the method comprising: providing an anode and a cathode, wherein each ofthe anode and the cathode comprises a uniform distribution of abifunctional electrocatalyst comprising metallic phosphides on aconductive substrate; and utilizing the anode and the cathode foralkaline water electrolysis, wherein the bifunctional electrocatalystpromotes hydrogen evolution reaction (HER) at the cathode, and oxygenevolution reaction (OER) at the anode.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and theadvantages thereof, reference is now made to the following briefdescription, taken in connection with the accompanying drawings anddetailed description, wherein like reference numerals represent likeparts.

FIG. 1a provides low-magnification SEM images of FeP/Ni₂P nanoparticlessupported on 3D Ni foam of Example 1;

FIG. 1B provides high-magnification SEM images of FeP/Ni₂P nanoparticlessupported on 3D Ni foam of Example 1;

FIG. 1c provides the SAED pattern taken from the FeP/Ni₂P catalysts ofExample 1;

FIG. 1d provides a typical HRTEM image taken from the FeP/Ni₂P catalystsof Example 1;

FIG. 1e shows the TEM image and corresponding EDX elemental mapping ofthe as-prepared samples of Example 1;

FIG. 1f is an XPS analysis of as-prepared samples of Example 1;

FIG. 1g is a typical XRD pattern of the samples of Example 1;

FIG. 1h is a typical TEM image of as-prepared FeP/Ni₂P nanoparticles ofExample 1;

FIG. 1i is an energy dispersive X-ray (EDX) spectrum of FeP/Ni₂Pnanoparticles of Example 1;

FIG. 2a shows XPS analysis of the original FeP/Ni₂P hybrid catalyst ofExample 1: Fe 2p^(3/2);

FIG. 2b shows XPS analysis of the original FeP/Ni₂P hybrid catalyst ofExample 1: Ni 2p^(3/2);

FIG. 2c shows XPS analysis of the original FeP/Ni₂P hybrid catalyst ofExample 1: O 1s;

FIG. 3a provides representative polarization curves, which show thegeometric current density plotted against applied potential vs.reversible hydrogen electrode (RHE) of the Fe—Ni—P hybrid electrode ofthis disclosure and Example 2 relative to Ni₂P and benchmark IrO₂catalysts;

FIG. 3b provides an enlarged region of the curves in FIG. 3 a;

FIG. 3c provides the corresponding Tafel plots for the catalysts ofExample 2;

FIG. 3d provides a comparison of the overpotentials required at 10 mAcm⁻² between the herein-disclosed FeP/Ni₂P catalyst and availablereported OER catalysts;

FIG. 3e provides a comparison of the current densities delivered at 300mV between the herein-disclosed FeP/Ni₂P catalyst and available reportedOER catalysts;

FIG. 3f shows the double-layer capacitance (C_(dl)) measurements of Ni₂Pand FeP/Ni₂P catalysts of Example 2;

FIG. 3g provides CV curves of FeP/Ni₂P before and after the accelerationdurability test for 5000 cycles of Example 2;

FIG. 3h provides the time-dependent potential curve for theherein-disclosed FeP/Ni₂P catalyst of Example 2 at 100 mA cm⁻²

FIG. 4a shows cyclic voltammetry (CV) curves (raw data) andcorresponding average activity calculated from the backward and forwardCV curves for Ni₂P, obtained with a scan rate of 1 mV s⁻¹;

FIG. 4b shows cyclic voltammetry (CV) curves (raw data) andcorresponding average activity calculated from the backward and forwardCV curves for FeP/Ni₂P, obtained with a scan rate of 1 mV s⁻¹;

FIG. 5a shows the scan rate dependence of the CV curves for Ni₂P as OERcatalyst, with scan rates ranging from 10 mV s⁻¹ to 100 mV s⁻¹ atintervals of 10 mV s⁻¹;

FIG. 5b shows the scan rate dependence of the CV curves for FeP/Ni₂P asOER catalyst, with scan rates ranging from 10 mV s⁻¹ to 100 mV s⁻¹ atintervals of 10 mV s⁻¹;

FIG. 6 shows Nyquist plots of electrochemical impedance spectroscopy(EIS) for OER catalyzed by Ni₂P and FeP/Ni₂P at an overpotential of 300mV;

FIG. 7a provides XPS analysis of the post-OER samples of Example 2: Fe2p region;

FIG. 7b provides XPS analysis of the post-OER samples of Example 2: Ni2p^(3/2) region;

FIG. 7c provides XPS analysis of the post-OER samples of Example 2: O 1sregion;

FIG. 7d provides XPS analysis of the post-OER samples of Example 2: P 2pregion;

FIG. 8 provides XRD patterns of the FeP/Ni₂P nanoparticles of Example 2after OER testing in comparison with that before the OER testing;

FIG. 9a provides the HER polarization curves of different catalysts ofExample 3;

FIG. 9b are Tafel plots for catalysts of FIG. 9a and Example 3;

FIG. 9c provides double-layer capacitance measurements for determiningelectrochemically active surface areas of Ni₂P and FeP/Ni₂P electrodesof Example 3;

FIG. 9d provides a comparison of the overpotentials required at 10 mAcm⁻² between the herein-disclosed FeP/Ni₂P catalyst of Example 3 andavailable reported HER catalysts;

FIG. 9e provides a comparison of the current densities delivered at −200mV between the herein-disclosed FeP/Ni₂P catalyst of Example 3 andavailable reported HER catalysts;

FIG. 9f provides polarization curves before and after the 5000 cyclingtest of Example 3;

FIG. 9g provides the chronopotentiometric curve of the herein-disclosedFeP/Ni₂P electrode of Example 3 tested at a constant current density of−100 mA cm⁻² for 24 h;

FIG. 9h provides the free energy diagram for ΔG_(H), the hydrogenadsorption free energy at pH=14 on the herein-disclosed FeP/Ni₂Pcatalyst of Example 3 in comparison with Ni₂P and benchmark Ptcatalysts;

FIG. 10 shows enlarged polarization curves of different HERelectrocatalysts of Example 3;

FIG. 11a provides the scan rate dependence of the current densities ofNi₂P/Ni foam, as HER catalyst with scan rates ranging from 1 mV s⁻¹ to10 mV s⁻¹ at intervals of 1 mV s⁻¹;

FIG. 11b provides the scan rate dependence of the current densities ofFeP/Ni₂P, as HER catalyst with scan rates ranging from 1 mV s⁻¹ to 10 mVs⁻¹ at intervals of 1 mV s⁻¹;

FIG. 12 provides Nyquist plots of Ni₂P and FeP/Ni₂P for HER measured at−150 mV vs. RHE;

FIG. 13 provides a comparison of the catalytic HER activity with thesame active surface area normalized by the C_(dl) difference betweenFeP/Ni₂P hybrid and pure Ni₂P* catalyst of Example 3;

FIG. 14 provides the CV curves recorded on the FeP/Ni₂P hybrid and pureNi₂P* electrodes in the potential ranges between −0.2 V vs. RHE and 0.6V vs. RHE in 1 M PBS;

FIG. 15a provides the molecular structures of the systems calculated inExample 3: Ni₂P(100);

FIG. 15b provides the molecular structures of the systems calculated inExample 3: FeP(001);

FIG. 15c provides the molecular structures of the systems calculated inExample 3: FeP(010);

FIG. 15d provides the molecular structures of the systems calculated inExample 3: Pt(111);

FIG. 15e provides the molecular structures of the systems calculated inExample 3: FeP(001) on Ni₂P(100), side view;

FIG. 15f provides the molecular structures of the systems calculated inExample 3: FeP(010) on Ni₂P(100), side view;

FIG. 16a provides the polarization curves of FeP/Ni₂P and IrO₂—Ptcoupled catalysts in a two-electrode configuration;

FIG. 16b shows an enlarged version at low current density region of FIG.16 a;

FIG. 16c provides a comparison of the cell voltages to achieve 10 mAcm⁻² among different water alkaline electrolyzers;

FIG. 16d provides a comparison of the cell voltages to achieve 100 mAcm⁻² among different water alkaline electrolyzers;

FIG. 16e provides a comparison of the current densities at 1.7 V for theherein-disclosed FeP/Ni₂P catalyst of Example 4 with available non-noblebifunctional catalysts;

FIG. 16f provides the catalytic stability of the FeP/Ni₂P catalysts ofExample 4 at 30, 100, and 500 mA cm⁻² for approximately 40 hours;

FIG. 17 provides a cyclic voltammetry (CV) curve (raw data; dashed) andcorresponding average activity (solid) calculated from the backward andforward CV curve of FeP/Ni₂P as a bifunctional catalyst for overallwater splitting obtained at a scan rate of 1 mV s⁻¹ in Example 4a;

FIG. 18a shows GC signals for the FeP/Ni₂P-based water alkalineelectrolyzer after 20 and 40 min of overall water splitting in Example4a; and

FIG. 18b shows the amounts of H₂ and O₂ gases versus time at a constantcurrent density of 100 mA cm⁻² in Example 4a.

DETAILED DESCRIPTION

It should be understood at the outset that although an illustrativeimplementation of one or more exemplary embodiments is provided below,the disclosed compositions, methods, and/or products may be implementedusing any number of techniques, whether currently known or in existence.The disclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated hereinbelow,including the exemplary designs and implementations illustrated anddescribed herein, but may be modified within the scope of the appendedclaims along with their full scope of equivalents.

The drawing figures are not necessarily to scale. Certain features andcomponents herein may be shown exaggerated in scale or in somewhatschematic form and some details of conventional elements may not beshown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and“comprising” are used in an open-ended fashion, and thus should beinterpreted to mean “including, but not limited to . . . .”

Overview

Discussed herein are methods of fabrication of an electrocatalystderived from metallic phosphides (e.g., iron phosphide (FeP) anddinickel phosphide (Ni₂P) (also referred to herein at times simply as‘nickel phosphide’) supported on a substrate (e.g., a conductive metalsubstrate such as commercial Ni foam), and the resultingelectrocatalyst, which is competent for catalyzing the overall watersplitting with outstanding catalytic performance. In embodiments, theherein-disclosed catalyst provides robust catalytic performance for boththe OER and HER, and outstanding performance for overall water splittingin base with good durability not only at low current density, but alsoat high current density (e.g., above 500 mA cm⁻²).

In embodiments, disclosed herein is a hybrid catalyst constructed byiron and dinickel phosphides on nickel foams that drives both hydrogenevolution reaction (HER) and oxygen evolution reaction (OER) well inbase. In embodiments, the hybrid catalyst of this disclosuresubstantially expedites overall water splitting at 10 mA cm⁻² (e.g.,with 1.42 V), and may outperform conventional, integrated IrO₂ and Ptcouple (with 1.57 V). In embodiments, the herein-disclosed catalystprovides excellent stability at high current densities (e.g., greaterthan or equal to about 500-1500 mA cm⁻²). For example, in embodiments,the herein-disclosed hybrid catalyst delivers 500 mA cm⁻² at lowoverpotential (e.g., less than or equal to about 1.72 V) with gooddurability (e.g., for at least 40 hours) without decay, providing greatpotential for large-scale applications.

Also disclosed herein is a method of hybridizing two metallic iron anddinickel phosphides (FeP/Ni₂P) on commercial nickel (Ni) foams toproduce an extremely active bifunctional electrocatalyst for both OERand HER, that provides exceptional overall water splitting surpassingconventional commercial alkaline electrolyzers in 1 M KOH. Inembodiments, an FeP/Ni₂P hybrid catalyst of this disclosure performswell for HER with catalytic performance (e.g., −14 mV to achieve −10mA/cm²) as good as that of the state-of-the-art noble Pt catalyst (−57mV), and also for OER with a very low overpotential (e.g., 154 mV toafford 10 mA/cm²), substantially outperforming the benchmark IrO₂ (281mV) and other known robust OER catalysts. Due to the excellent HER andOER activity of the herein-disclosed bifunctional electrocatalyst, inembodiments, a bifunctional catalyst of this disclosure can be utilizeddirectly as both the anode and cathode electrodes in analkaline-electrolyzer. In such embodiments, a low cell voltage (e.g.,1.42 V) can deliver 10 mA/cm², and a low cell voltage (e.g., 1.72 V) candeliver 500 mA cm⁻² with 40 h durability. The herein-disclosedbifunctional electrocatalyst may thus surpass the performance ofconventional industrial catalysts, which typically require 2.40 V for400 mA/cm².

Method of Making Electrocatalyst/Electrode

As noted above, herein-disclosed is a method of manufacturing anelectrode/electrocatalyst for hydrogen evolution reaction (HER) at thecathode and oxygen evolution reaction (OER) at the anode in overallwater splitting. In embodiments, the catalyst is a three-dimensionalFe-mainly bifunctional electrocatalyst. In embodiments, the (e.g.,iron-mainly) electrocatalyst is directly grown on Ni foam or otherconductive scaffolds, such as, without limitation, carbon cloth paper,Cu foam, Co foam, Fe foam, Ti foam, or a combination thereof. Theelectrocatalyst may be grown, in embodiments, via a two-time thermalphosphidation in a tube furnace using iron-based aqueous solutions and Psources (e.g., iron nitrate and red phosphorus, respectively). Via theherein-disclosed method, a hybrid FeP/Ni₂P catalyst with high porosityon Ni foam surface can be prepared.

Herein-disclosed is a method of manufacturing a bifunctionalelectrocatalyst for overall water splitting comprising oxygen evolutionreaction (OER) and hydrogen evolution reaction (HER). In embodiments,the method comprises growing electrocatalyst comprising primarilymetallic phosphides on a three-dimensional substrate. Theelectrocatalyst comprising primarily metallic phosphides can be grown onthe three-dimensional substrate by immersing the substrate in an ironnitrate solution to form a once disposed substrate; subjecting the oncedisposed substrate to (e.g., a first) thermal phosphidation with redphosphorus under inert gas to grow metal phosphides thereupon and form aonce subjected substrate; and cooling the once subjected substrate toform a cooled, once subjected substrate comprising the bifunctionalelectrocatalyst comprising metallic phosphides disposed on thethree-dimensional substrate. In embodiments, the method can comprise atwo-time thermal phosphidation. In such embodiments, the method canfurther comprise immersing the cooled, once subjected substrate in aniron nitrate solution to form a twice disposed substrate; and subjectingthe twice disposed substrate to (e.g., a second) thermal phosphidationwith red phosphorus under inert gas to provide an electrode comprisingthe bifunctional electrocatalyst on the three-dimensional substrate.Accordingly, in embodiments, a method of manufacturing a bifunctionalelectrocatalyst for overall water splitting comprising oxygen evolutionreaction (OER) and hydrogen evolution reaction (HER) comprises growingelectrocatalyst comprising primarily metallic phosphides on athree-dimensional substrate by immersing the substrate in an ironnitrate solution to form a once disposed substrate, subjecting the oncedisposed substrate to thermal phosphidation with phosphorus under inertgas to grow metal phosphides thereupon and form a once subjectedsubstrate, cooling the once subjected substrate to form a cooled, oncesubjected substrate, immersing the cooled, once subjected substrate inan iron nitrate solution to form a twice disposed substrate, andsubjecting the twice disposed substrate to thermal phosphidation withphosphorus under inert gas to provide an electrode comprising thebifunctional electrocatalyst on the three-dimensional substrate.

In embodiments, the three-dimensional substrate can comprise anysuitable conductive scaffold. The three-dimensional substrate cancomprise a conductive foam. In embodiments, the substrate comprises ametal foam. In embodiments, the three-dimensional substrate comprisesone or more of a metallic foam or a carbon cloth paper. In embodiments,the metallic foam comprises nickel (Ni) foam, copper (Cu) foam, iron(Fe) foam, cobalt (Co) foam, titanium (Ti) foam, or a combinationthereof. For example, in embodiments, the substrate comprises nickel(Ni) foam. The foam of the substrate, in embodiments, can have anysuitable thickness. For example, in embodiments, the foam has athickness in the range of from about 1 mm to about 2 mm. In embodiments,the foam may have a purity of at least 99.8%. In embodiments, the foamcomprises a nickel foam. In embodiments, the nickel foam can have asurface density in the range of from about 280 g/m² to about 340 g/m².In embodiments, the nickel foam can have a porosity of greater than orequal to about 95, 96, or 97%, or in the range of from about 95 to about97%, and may comprise from about 80 to about 110 pores per inch, and/oraverage pore diameters in the range of from about 0.2 to about 0.6 mm.

The iron nitrate solution can be selected from iron(III)nitrate(Fe(NO₃)₃), iron(II)nitrate (Fe(NO₃)₂), or a combination thereof. Inembodiments, the iron nitrate solution comprises iron(III) nitratenonahydrate [Fe(NO₃)₃.9H₂O], iron(III)nitrate (Fe(NO₃)₃), or acombination thereof. Alternatively or additionally, iron oxide or(oxy)hydroxide nanostructures including nanoparticles, nanosheet ornanowire arrays may be, in embodiments, pre-loaded on the surface ofconductive supports (e.g., Ni foam, Cu foam, etc.) by electrodepositionor physical dipping.

In embodiments, the electrode comprises a nickel (Ni) foam with dinickelphosphide (Ni₂P) and iron phosphide (FeP) formed on the surface of thenickel foam substrate. In such embodiments, therefore, the electrode cancomprise a three-dimensional, porous FeP/Ni₂P/Ni foam.

In embodiments the thermal phosphidation (e.g., the first thermalphosphidation, the second thermal phosphidation, or both) is effected ata temperature in the range of from about 300° C. to about 550° C., fromabout 400° C. to about 550° C., from about 400° C. to about 450° C., orfrom about 300° C. to about 350° C. In embodiments, the thermalphosphidation (e.g., the first thermal phosphidation, the second thermalphosphidation, or both) is effected at a temperature of less than orequal to about 400° C., 425° C., or 450° C. In embodiments, thermalphosphidation (e.g., the first thermal phosphidation, the second thermalphosphidation, or both) is effected in a total time of less than orequal to about 1.5 hours, 1.25 hours, or 1 hour. In embodiments,subjecting the once disposed substrate to thermal phosphidation,subjecting the twice disposed substrate to thermal phosphidation, orboth comprises a thermal phosphidation for a duration of time in therange of from about 1 hour to about 4 hours, from about 1 hour to about2 hours, or from about 2 hours to about 4 hours. Subjecting to thermalphosphidation can comprise direct thermal phosphidation via any meansknown in the art, for example, in a tube furnace or a chemical vapordeposition (CVD) system or molecular organic chemical vapor deposition(MOCVD) system under inert gas (e.g., under argon atmosphere).

The inert gas under which thermal phosphidation and/or cooling iseffected can comprise argon (e.g., substantially pure argon), inembodiments. The phosphorus utilized during thermal phosphidation cancomprise phosphorus powder, for example, red phosphorus, or PH₃ gas fromsodium hypophosphite (NaH₂PO₂) with a very slow heating rate forphosphidation, or a combination thereof. During the first or secondthermal phosphidation, red phosphorus powder can be used as thephosphorus source with a controlled temperature for heating, in which nowater gas may be generated during phosphorus vapor going out. Thereby,phosphorus vapor can convert the iron precursors or metallic nickel tometal phosphides, rather than metal metaphosphate.

In embodiments, the metallic phosphides comprise primarily a combinationof iron phosphide and dinickel phosphide. In embodiments, of theprimarily iron and dinickel phosphides, the metallic phosphides comprisea majority of iron phosphide (FeP) and a minority of nickel phosphide(Ni₂P). ‘Primarily’ iron and dinickel phosphides indicates that, of thetotal metallic phosphides, more than at least 50 weight percentcomprises iron and dinickel phosphides. In embodiments, greater than orequal to about 50, 60, 70, 80, or 90 weight percent of the totalmetallic phosphides comprise iron and dinickel phosphides. The‘majority’ of iron phosphide (FeP) means that, based on the total ironand dinickel phosphides, greater than 50 weight percent comprises ironphosphide. In embodiments, greater than or equal to 50, 60, 70, 80, 85,86, or 87 weight percent of the iron and dinickel phosphides compriseiron phosphide. The ‘minority’ of dinickel phosphide (Ni₂P) means that,based on the total iron and dinickel phosphides, less than 50 weightpercent comprises dinickel phosphide. In embodiments, less than or equalto 20, 15, or 12.5 weight percent of the iron and dinickel phosphidescomprise dinickel phosphide. In embodiments, the metallic phosphidescomprise less than or equal to about 20, 15, 14, 13, or 12.5% weightpercent dinickel phosphide (Ni₂P) and greater than or equal to about 80,81, 82, 83, 84, 85, 86, 87, or 87.5% weight percent iron phosphide(FeP). In embodiments, the three-dimensional substrate comprises nickel(Ni) foam, the metal phosphides of the bifunctional electrocatalystcomprise or consist essentially of FeP and Ni₂P, and the electrodecomprises or consists essentially of an FeP/Ni₂P/Ni foam.

In embodiments, the method can further comprise drying. The drying canbe performed subsequent disposing the three-dimensional substrate in themetal nitrate solution. In embodiments, drying comprises drying inambient air.

Cooling can comprise cooling under inert gas, such as argon atmosphere.If a second thermal phosphidation step is performed, the steps ofimmersing the cooled, once subjected substrate in an iron nitratesolution to form a twice disposed substrate, and subjecting the twicedisposed substrate to thermal phosphidation may be performed asdescribed herein, and may be performed in substantially the same or adifferent manner from the steps followed the first time for immersingthe substrate in the iron nitrate solution to form the once disposedsubstrate, subjecting the once disposed substrate to thermalphosphidation with phosphorus under inert gas to grow metal phosphidesthereupon and form the once subjected substrate. For example, during therepeating of the step(s), the metal nitrate solution may be the same ordifferent, drying may be utilized or omitted following immersing thesubstrate, the thermal phosphidation may be effected for a longer orshorter time period, at a higher or lower temperature, with a same ordifferent phosphorus source or inert gas, etc. A second cooling step mayfollow the second thermal phosphidation step, in embodiments.

Herein-Disclosed Electrocatalyst/Electrode

Also disclosed herein is a bifunctional electrocatalyst and electrodeproduced as described hereinabove. In embodiments, the bifunctionalelectrocatalyst has a high porosity. In embodiments, the electrocatalystis operable for alkaline water electrolysis. In embodiments, thebifunctional electrocatalyst exhibits good performance for both the HERand the OER, and is stable at current densities of up to at least 100mA/cm², 300 mA/cm², or 500 mA/cm².

As noted hereinabove, in embodiments, the metallic phosphides of thebifunctional catalyst formed on the substrate can comprise ironphosphide (FeP) and dinickel phosphide (Ni₂P), or a combination thereof.In embodiments, a catalyst loading of the metallic phosphides on thesubstrate can be in the range of from about 8 to about 15 mg/cm².

Also disclosed herein is an electrode for overall water splitting, theelectrode comprising: a substrate; and a bifunctional electrocatalystcomprising primarily metallic phosphides on a surface of the substrate.As noted above, the substrate of the herein-disclosed electrode cancomprise a three dimensional substrate, such as, without limitation, ametal foam or carbon cloth paper. The metal foam can comprise nickel(Ni) foam, copper (Cu) foam, iron (Fe) foam, cobalt (Co) foam, titanium(Ti) foam, or a combination thereof, in embodiments. In embodiments, themetal foam comprises nickel (Ni) foam.

In embodiments, the metallic phosphides of the herein-disclosedelectrode comprise primarily iron phosphide (FeP) and nickel phosphide(Ni₂P). In embodiments, the metallic phosphides comprise a very smallfraction of nickel phosphide (Ni₂P) and a majority of iron phosphide(FeP). In embodiments, a loading of the bifunctional electrocatalystcomprising primarily metallic phosphides on the substrate of theherein-disclosed electrode is in the range of from about 8 to about 15mg/cm². In embodiments, a loading of dinickel phosphide (Ni₂P) on thesubstrate of the herein-disclosed electrode is in the range of fromabout 1 to about 2 mg/cm², or less than or equal to about 2, 1.5, or 1mg/cm². In embodiments, a loading of iron phosphide (FeP) on thesubstrate of the herein-disclosed electrode is in the range of fromabout 7 to about 13 mg/cm², or greater than or equal to about 7, 8, 9,10, 11, 12, or 13 mg/cm².

In embodiments, the three dimensional substrate comprises nickel (Ni)foam, the metallic phosphides comprise primarily nickel phosphide (Ni₂P)and iron phosphide (FeP), and the herein-disclosed electrode thuscomprises or consists essentially of FeP and Ni₂P on Ni foam.

In embodiments, the bifunctional electrocatalyst of the herein-disclosedelectrode has a high porosity, which may, in embodiments, include aporosity of greater than or equal to a porosity of the three-dimensionalsubstrate.

In embodiments, when operated in 1M alkaline solution, the bifunctionalelectrocatalyst/electrode of this disclosure requires a lowoverpotential of less than 15 mV and 155 mV, respectively, to deliver acurrent density of 10 mA/cm² for the HER and OER, and/or may be operableto provide to an overall water-splitting activity at 10 mA/cm² with lessthan 1.45 V.

In embodiments, when operated in 1M alkaline solution, the bifunctionalelectrocatalyst/electrode of this disclosure yields a current density ofat least 100 mA/cm² at an overpotential of less than or equal to about225 mV for the OER, exhibits durability for at least 5,000 cycles, isoperable for at least 40 hours at 100 mA/cm², or a combination thereof.

Herein-Disclosed Method of Electrocatalytic Water Splitting

Also disclosed herein is a method of electrocatalytic water splitting,the method comprising: providing or forming an anode and a cathode,wherein at least one or both of the anode and the cathode comprise auniform distribution of a bifunctional electrocatalyst of thisdisclosure (e.g., comprising metallic phosphides) on a conductivesubstrate; and utilizing the anode and the cathode for alkaline waterelectrolysis, wherein the bifunctional electrocatalyst promotes hydrogenevolution reaction (HER) at the cathode, and oxygen evolution reaction(OER) at the anode.

In embodiments, the bifunctional catalyst of the anode has the samecomposition as the bifunctional catalyst of the cathode. In embodiments,the metallic phosphides of the anode and/or the cathode compriseprimarily iron phosphide (FeP) and nickel phosphide (Ni₂P). Inembodiments, the metallic phosphides of the anode and/or the cathodecomprise a majority of iron phosphide (FeP) and a minority of nickelphosphide (Ni₂P). In embodiments, a loading of the metallic phosphideson the conductive substrate of the anode and/or the cathode is in therange of from about 8 to about 15 mg/cm². In embodiments, a loading ofdinickel phosphide (Ni₂P) on the conductive substrate of the anodeand/or the cathode is in the range of from about 1 to about 2 mg/cm². Inembodiments, a loading of dinickel phosphide (Ni₂P) on the conductivesubstrate of the anode and/or the cathode is less than or equal to about2, 1.5, 1 mg/cm². In embodiments, a loading of iron phosphide (FeP) onthe conductive substrate of the anode and/or the cathode is in the rangeof from about 7 to about 13 mg/cm². In embodiments, a loading of ironphosphide (FeP) on the conductive substrate of the anode and/or thecathode is greater than or equal to about 7, 8, 9, 10, 11, 12, or 13mg/cm².

The conductive substrate of the anode and/or the cathode can comprisenickel foam, in embodiments. In embodiments, the anode and/or thecathode comprise an FeP/Ni₂P/Ni foam. In embodiments, when operated in1M alkaline solution, the bifunctional electrocatalyst of the anodeand/or the cathode requires a low overpotential of less than 15 mV and155 mV, respectively, to deliver a current density of 10 mA/cm² for theHER and OER, leading to an overall water-splitting activity at 10 mA/cm²with less than 1.45 V.

In embodiments, the bifunctional electrocatalyst of the anode and/or thecathode has a high porosity, as evidenced, in embodiments, by a porosityof the bifunctional electrocatalyst on the conductive substrate of theanode and/or the cathode, respectively, that is greater than that of theconductive support.

Performance. In embodiments, the strong synergistic effects between thephosphides (e.g., between FeP and its support Ni₂P), good electricalconductivity of the substrate (e.g., Ni foam) and the metal phosphides,and high porosity of a herein-disclosed bifunctional (e.g., FeP/Ni₂P)electrocatalyst contribute greatly to the outstanding HER and OERactivities of the final (e.g., FeP/Ni₂P) catalyst. For example, inembodiments, a herein-disclosed FeP/Ni₂P catalyst requires relativelylow overpotentials of 14 mV and 154 mV to deliver a current density of10 mA cm⁻² for the HER and OER in base, respectively, leading to robustoverall-water-splitting activity at 10 mA cm⁻² with 1.42 V. Thisactivity outperforms the conventional integrated IrO₂ and Pt couple(1.57 V), demonstrating that Fe compounds are promising materials forwater splitting, especially for catalyzing the OER process. Inembodiments, the herein-disclosed bifunctional electrocatalysts canprovide exhibit excellent durability without decay operated at highcurrent densities above 500 mA cm⁻², providing great potential forlarge-scale applications.

Features and Potential Advantages

Herein disclosed is a rational design of a high performance bifunctionalcatalyst for overall water splitting. In embodiments, the catalyst is aporous, bifunctional FeP/Ni₂P electrocatalyst. In embodiments,herein-disclosed is an FeP/Ni₂P hybrid catalyst supported on 3D Ni foamthat proves to be an outstanding bifunctional catalyst for overall watersplitting, exhibiting both extremely high OER and HER activities in thesame alkaline electrolyte. In embodiments, the herein-disclosed FeP/Ni₂Phybrid catalyst supported on 3D Ni foam sets a new record in alkalinewater electrolyzers (e.g., 1.42 V to afford 10 mA/cm²), while at thecommercially practical current density of 500 mA/cm² demanding a voltageof only 1.72 V, lower than those for any previously reportedbifunctional catalysts, and maintains excellent catalytic activity formore than 40 hours at a current density of 500 mA/cm², paving the wayfor promising large-scale hydrogen generation.

Considering the low cost and earth abundance of the compounds utilizedto produce the herein-disclosed catalyst, these catalysts may bepromising alternatives to the noble catalysts for the OER, such as IrO₂catalysts and Ni alloy-based electrocatalysts currently used incommercial alkaline water electrolysis. In particular, commerciallypurchased Ni foam as well as other conductive scaffolds can be suppliedin large scale, so the as-prepared hybrid catalysts of this disclosurecan be compatible, in embodiments, with sizable electrodes for potentialapplications in water electrolysis.

EXAMPLES

The embodiments having been generally described, the following examplesare given as particular embodiments of the disclosure and to demonstratethe practice and advantages thereof. It is understood that the examplesare given by way of illustration and are not intended to limit thespecification or the claims in any manner.

Chemicals

Red phosphorous powder (Sigma Aldrich, ≥97%, CAS No. 7723-14-0),Iron(III) nitrate nonahydrate [Sigma Aldrich, Fe(NO₃)₃.9H₂O, ≥99.95%,CAS No. 7782-61-8], Pt wire (CH Instruments, Inc.), Nafion 117 solution(5%; Sigma-Aldrich), iridium oxide powder (Alfa Aesar, IrO₂, 99%),potassium hydroxide (Alfa Aesar, KOH, 50% wt./vol.), and Ni foam (arealdensity 320 g/cm²) were used without further purification.

Methods

Material synthesis. Metal phosphides (Ni₂P and FeP) were grown bychemical vapor deposition in a tube furnace, in which Ni foam, Fe(NO₃)₃,and phosphorus were utilized as the Ni, Fe, and P sources, respectively.Namely, a commercially hydrophobic Ni foam was immersed into an aqueousFe(NO₃)₃ solution, which was then converted to mainly FeP and a verysmall fraction of Ni₂P at 450° C. in argon (Ar) atmosphere, withphosphorus powder supplied upstream. After that, the samples werenaturally cooled down under Ar gas protection, which became hydrophilicafter first phosphidation. In the following, a second-time phosphidationwas performed after the samples were immersed into the Fe(NO₃)₃ solutionagain. For comparison, the as-prepared Ni₂P samples were obtained in thesame growth conditions without the addition of Fe(NO₃)₃, and the Ni₂P*samples were grown in the same experimental conditions by using Ni(NO₃)₂instead of Fe(NO₃)₃. Thus, Ni₂P* is a reference catalyst wherein theFe(NO₃)₃ is replaced by Ni(NO₃)₂ for growing Ni₂P on the Ni foam surfaceby the same experimental conditions; the asterisk (“*”) is utilized todenote that this Ni₂P* is different from Ni₂P samples naturally grownwithout using Ni(NO₃)₂.

Electrochemical Characterization. The electrochemical tests wereperformed via a typical three-electrode configuration in 100 mL 1 M KOHelectrolyte. Polarization curves for the HER were recorded by linearsweep voltammetry with a scan rate of 1.0 mV s⁻¹. For the OER andoverall water splitting, in order to minimize the effect of capacitivecurrent originating from the Ni ions oxidation on the catalyticperformance, cyclic voltammetry (CV) curves with the forward andbackward sweeps with a very small scan rate of 1 mV s⁻¹ were utilized tocalculate the average activity. A carbon paper was used as the counterelectrode for both the HER and OER tests. The scan rate for the cyclingtests was set to 50 mV s⁻¹. The potentials were converted to reversiblehydrogen electrode (RHE).

Computational Methods. At the generalized gradient approximation (GGA)level of density functional theory (DFT) was employed to calculate therelative energies of relevant structures. More specifically,Perdew-Burke-Ernzerhof (PBE) functional with a D3 correction was usedfor both geometry optimizations and the single point free energies.Geometry optimizations were performed in the Vienna Ab initio SimulationPackage (VASP) with projected augmented wave (PAW) and VASPsol solvationmethod. Kinetic energy cutoff for geometry optimization is 300 eV and,for single point energy it is 13 Hartree (3.54 eV). Single point freeenergies are calculated in jDFTx with charge-asymmetricnonlocally-determined local-electric solvation model (CANDLE) implicitsolvation and Garrity-Bennett-Rabe-Vanderbilt ultra-softpseudopotentials (GBRV uspp). The final free energy G was calculated asG=F−n_(e)U+ZPE+H_(vib)−TS_(vib), where F is the energy of the solvatedKohn-Sham DFT electronic system, n_(e) is the number of electrons in thesystem, and U is the chemical potential for elections. jDFTx and VASPsolare solution packages built on joint DFT and VASP, respectively.

Results Example 1: Electrocatalyst Synthesis and Characterization

An Fe—Ni—P hybrid architecture was prepared directly on commercial Nifoams by a thermal treatment process. Following synthesis, microscopiccharacterization of as-prepared FeP/Ni₂P hybrid included scanningelectron microscopy (SEM). FIG. 1a and FIG. 1B provide low- andhigh-magnification SEM images of FeP/Ni₂P nanoparticles supported on 3DNi foam. The typical scanning electron microscopy (SEM) images of FIG.1a and FIG. 1B show that the as-prepared samples are free-standing withabundant mesopores and/or nanopores at the surface, indicatingefficacious achievement of large surface areas for facile exchange ofproton or oxygen-containing intermediates. In particular, numerousnanocrystals are distributed uniformly at the surface, forming plentifulsurface active sites in this hybrid catalyst. FIG. 1c and FIG. 1dprovide the SAED pattern and a typical HRTEM image, respectively, takenfrom the FeP/Ni₂P catalysts. FIG. 1h is a typical TEM image ofas-prepared FeP/Ni₂P nanoparticles. The selected area electrondiffraction (SAED) pattern (FIG. 1c ), combined with high-resolutiontransmission electron microscopy (TEM) images (FIG. 1d , FIG. 1h ),further reveal the nanoscale features of the FeP and Ni₂P particles withdiameters of 5-30 nm. The interplanar spacings of these nanoparticlesare resolved by TEM to be around 0.204 and 0.502 nm corresponding to the(021) and (010) planes of Ni₂P crystals, and 0.181 and 0.193 nmcorresponding to the (103) and (220) planes of FeP crystals.

To determine the distribution of Ni, Fe, and P elements in theas-prepared samples, elemental mapping was carried out using TEM,confirming the homogenous distribution of Ni, Fe, and P elements in theFeP/Ni₂P nanoparticles. FIG. 1e shows the TEM image and correspondingEDX elemental mapping of the as-prepared samples. FIG. 1i is an energydispersive X-ray (EDX) spectrum of FeP/Ni₂P nanoparticles. The energydispersive X-ray (EDX) spectrum of FIG. 1i shows that the Ni, Fe, and Pelements are present with an atomic ratio close to 2:1:2, consistentwith the high-resolution TEM observations.

The chemical composition and oxidation states of the catalysts werefurther unveiled by X-ray photoelectron spectroscopy (XPS) and X-raydiffraction (XRD). As seen in FIG. 1f , which is an XPS analysis, the P2p core level spectrum can be fit with two doublets, with one located atthe binding energies of 129.3 and 130.1 eV attributing to phosphorusanions of metal phosphides, and the other at 133.5 and 134.3 eVindicative of phosphate-like P arisen from possible surface oxidation.FIGS. 2a-c show XPS analysis of the original FeP/Ni₂P hybrid catalyst(a) Fe 2p^(3/2), (b) Ni 2p^(3/2), and (c) O 1s, respectively. As seen inFIG. 2a , the XPS spectrum of Fe 2p3/2 core level can be deconvolutedinto three main peaks with binding energies of 707.0, 709.9, and 711.9eV assigned to FeP, Fe-based oxide and phosphate, respectively, causedby possible superficial oxidation when exposing FeP samples to air,while another peak located at 714.3 eV is arisen from the relevantsatellite peak. As seen in FIG. 2b , this peak deconvolution is alsoapplied to the Ni 2p³¹² core level spectrum, where three bindingenergies located at 853.6, 856.4, and 861.0 eV are ascribed to Ni₂P,Ni—PO_(x), and the corresponding satellite peak, respectively. As seenin FIG. 1g , which is a typical XRD pattern of the samples (to betterview the peaks from the catalysts, the full intensity of the peaks fromNi are not shown in FIG. 1g ), the typical XRD pattern reveals the mainindexes from the as-prepared FeP/Ni₂P hybrid and Ni foam support. Thetwo strongest peaks at 45° and 52° are mainly originated from the Nifoam support (ICSD-53809). The other peaks are those characteristic ofFeP (ICSD-633046) and Ni₂P (ICSD-646102), consistent with the TEManalysis.

Example 2: Oxygen Evolution Catalysis

The catalytic OER activity of the Fe—Ni—P hybrid catalyst of Example 1was evaluated in 1.0 M KOH aqueous electrolyte. FIG. 3a and FIG. 3bprovide representative polarization curves which show the geometriccurrent density plotted against applied potential vs. reversiblehydrogen electrode (RHE) of the Fe—Ni—P hybrid electrode relative toNi₂P and benchmark IrO₂ catalysts. FIG. 3b provides the enlarged regionof the curves in FIG. 3a . FIG. 4a and FIG. 4b show a cyclic voltammetry(CV) curve (raw data) and corresponding average activity calculated fromthe CV curve for Ni₂P and FeP/Ni₂P, respectively, obtained with a scanrate of 1 mV s⁻¹. The effect of capacitive current on the catalyticactivity, originating from the Ni ions oxidation, is minimized bycalculating the average activity from the forward and backward sweeps ofa cyclic voltammetry (CV) curve. Strikingly, as seen from FIG. 4a andFIG. 4b , the Fe—Ni—P hybrid requires an overpotential of only 154 mV todeliver 10 mA/cm², which is 127 mV less than the state-of-the-art IrO₂catalyst (281 mV). At 281 mV, the herein-disclosed FeP/Ni₂P catalystachieves a current density up to 690 mA/cm², which is 69-fold higherthan the benchmark IrO₂, demonstrating a substantial improvement of theOER activity. Indeed, as seen in Table 1 below, this overpotential of154 mV in alkaline conditions is among the lowest for catalyzing OERthus far, even surpassing the presently most active NiFe LDH (doublelayered hydroxide) catalyst (˜200 mV).

Table 1 provides a comparison of catalytic performance of theherein-disclosed FeP/Ni₂P electrocatalyst with the most recentlyreported OER catalysts. In Table 1, η_(10,OER) corresponds to theoverpotential of OER catalyzed at a current density of 10 mA cm⁻², whilej_(300,OER) corresponds to the current density at 300 mV overpotentialfor the OER.

TABLE 1 Comparison of Catalytic Performance of FeP/Ni₂P Electrocatalystwith Reported OER Catalysts η_(10, OER) Tafel OER catalysts Electrolytes(mV) (mV dec⁻¹) Source FeP/Ni₂P 1.0M KOH 154 22.7 Herein-DisclosedGelled FeCoW 1.0M KOH 191 22.7 Reference Ni_(x)Fe_(1−x)Se₂-DO 1.0M KOH195 28 Reference NiCeO_(x)—Au 1.0M NaOH 270 — Reference Ni₂Pnanoparticles 1.0M KOH 290 59 Reference Co₄N 1.0M KOH 257 44 Referenceh-NiS_(x) 1.0M KOH 180 96 Reference FeP-rGO 1.0M KOH 260 49.6 ReferenceBifunctional catalysts j_(300, OER) for the OER Electrolytesη_(10, OER (mV)) (mA cm⁻²) Source FeP/Ni₂P 1.0M KOH 154 1277  Herein-Disclosed Porous MoO₂ 1.0M KOH 260 41* ReferenceNi_(0.51)Fe_(0.49)P film 1.0M KOH 239 80* Reference MoS₂/Ni₃S₂ 1.0M KOH218 100*  Reference CoP₂/rGO 1.0M KOH 300 10  Reference NiCo₂S₄ nanowirearray 1.0M KOH 260 19* Reference Electrodeposited CoP film 1.0M KOH 345  0.5* Reference NiCo₂O₄ 1.0M KOH 290 24* ReferenceEG/Co_(0.85)Se/NiFe-LDH 1.0M KOH 206 300*  Reference NiFe LDH 1.0M KOH240 30* Reference NiFe LDH@DG10 1.0M KOH 201   77.5* Reference NiFeLDH/Cu NW 1.0M KOH 199 214*  Reference NiFeO_(x)/CFP 1.0M KOH 230 400* Reference NiP/Ni 1.0M KOH 247 50* Reference

FIG. 3c provides the Tafel plots corresponding to the data in FIG. 3aand FIG. 3b . As seen in FIG. 3c , a very small Tafel slope of 22.7 mVdec⁻¹ was measured, which is much smaller than those of the referencematerials Ni₂P (102.3 mV dec⁻¹) and IrO₂ (71.7 mV dec⁻¹), and alsosmaller than most of the OER catalysts reported, for example, inTable 1. Specifically, the OER activity of the herein-disclosed FeP/Ni₂Pcatalyst was compared with other available bifunctional catalysts asshown in FIG. 3d and FIG. 3e , which provide a comparison of theoverpotentials required at 10 mA cm⁻² between the herein-disclosedcatalyst and available reported OER catalysts and a comparison of thecurrent densities delivered at 300 mV between the herein-disclosedcatalyst and available reported OER catalysts, respectively. From FIG.3d and FIG. 3e , it is evident that the herein-disclosed FeP/Ni₂Pcatalyst requires the lowest overpotential (e.g., 154 mV) to achieve 10mA/cm², and the largest current density (1277 mA/cm²) at 300 mVoverpotential, indicating the potential to be used for overall watersplitting with large current densities at small cell voltage.

To elucidate the origins of the remarkably high OER catalytic activityof the herein-disclosed FeP/Ni₂P catalyst, electrochemical impedancespectroscopy (EIS) and double-layer capacitance (Cdl) investigationswere performed on this FeP/Ni₂P electrode. FIG. 5a and FIG. 5b show thescan rate dependence of the current densities for Ni₂P and FeP/Ni₂P asOER catalysts, respectively, with scan rates ranging from 10 mV s⁻¹ to100 mV s⁻¹ at intervals of 10 mV s⁻¹. FIG. 3f shows the double-layercapacitance (C_(dl)) measurements of Ni₂P and FeP/Ni₂P catalysts. Asseen in FIG. 3f , this capacitance C_(dl) determined by a simple cyclicvoltammetry method (FIG. 5) is calculated to be 19.3 mF/cm² for theFe—Ni—P hybrid electrode, very close to that of the Ni₂P catalyst (14.5mF/cm²). This manifests that depositing FeP on the Ni₂P surface doesn'tresult in huge changes in the active surface area, while theelectrochemical OER performance of FeP/Ni₂P is much better than Ni₂P.For instance, the herein-disclosed FeP/Ni₂P hybrid achieves 1000 mA/cm²at 293 mV, while Ni₂P can deliver only 32 mA/cm² at this overpotential,making our FeP/Ni₂P catalyst ˜30-fold better than the Ni₂P catalyst,heralding that unexpected synergistic effects between FeP and Ni₂P inthe hybrid is the main contributor to the superior catalytic performanceprovided thereby, not just the high active surface area. FIG. 6 showsNyquist plots of electrochemical impedance spectroscopy (EIS) for OERcatalyzed by Ni₂P and FeP/Ni₂P at an overpotential of 300 mV. As seen inFIG. 6, the EIS spectra show that the herein-disclosed FeP/Ni₂P hybridhas a lower charge-transfer resistance at the interface of the catalystswith Ni foam, leading to faster OER kinetics compared to the Ni₂Pcatalyst. Therefore, without wishing to be limited by theory, theexcellent OER activity of the herein-disclosed FeP/Ni₂P hybrid catalystmay be attributed to fast electron transport and synergistic effectsbetween FeP and Ni₂P.

Electrochemical durability is another key index that can be utilized toevaluate the performance of electrocatalysts. FIG. 3g provides CV curvesof FeP/Ni₂P before and after the acceleration durability test for 5000cycles. Obviously, as seen in FIG. 3g , after 5000 cycling test, the CVcurve of the FeP/Ni₂P hybrid of this disclosure is nearly identical tothe original one, suggesting its excellent durability during cyclingscans. The long-term electrochemical stability of the catalyst tested at100 mA/cm² was probed. FIG. 3h provides the time-dependent potentialcurve for the herein-disclosed FeP/Ni₂P catalyst at 100 mA cm⁻². As seenin FIG. 3h , the real-time potential remained nearly constant during a24 h continuous operation. These results establish the strong durabilityof the herein-disclosed FeP/Ni₂P catalyst for OER in alkalineelectrolyte. Further insights into the chemical compositions forpost-OER samples were determined by XPS and XRD. FIGS. 7a-7d provide XPSanalysis of the post-OER samples: FIG. 7a -Fe 2p region; FIG. 7b Ni2p^(3/2) region; FIG. 7c —O 1s region; and FIG. 7d -P 2p region. FIG. 8provides XRD patterns of the FeP/Ni₂P nanoparticles after OER testing.Without wishing to be limited by theory, from the XPS spectra of FIGS.7a-7d and the XRD patterns of FIG. 8, it appears that a mixture ofnickel and iron oxides/oxyhydroxides evolves at the surface of theFeP/Ni₂P hybrid during OER.

Example 3: Hydrogen Evolution Catalysis

Electrocatalytic hydrogen evolution reaction in 1 M KOH electrolyteusing the herein-disclosed FeP/Ni₂P electrocatalyst was studied. Inaddition to the excellent OER performance, the herein-disclosed FeP/Ni₂Phybrid is highly active towards HER in the same electrolyte. FIG. 9aprovides the HER polarization curves of different catalysts. FIG. 10shows enlarged polarization curves of different HER electrocatalysts.From FIG. 9A and FIG. 10, it is evident that the bare Ni₂P is not a goodHER catalyst, as it requires a large overpotential of −150 mV to delivera current density of −10 mA/cm². Conversely, the herein-disclosedFeP/Ni₂P hybrid unexpectedly obtains −10 mA/cm² at an extremely lowoverpotential of −14 mV. As seen in Table 2, which provides a comparisonof catalytic performance of the herein-disclosed FeP/Ni₂P catalyst withavailable non-noble HER catalysts in alkaline electrolytes, this lowoverpotential of −14 mV is the lowest value among non-noble metal-basedHER catalysts, and indeed is comparable to that of Pt (−59 mV) inalkaline electrolyte. In Table 2, η_(10, HER) corresponds to theoverpotential of HER catalyzed at 10 mA cm⁻², and j_(200, HER) isrelated to the current density at 200 mV overpotential.

TABLE 2 Comparison of Catalytic Performance with Available Non-Noble HERcatalysts in Alkaline Electrolytes η_(10, HER) Tafel HER catalystsElectrolytes (mV) (mV dec⁻¹) Source FeP/Ni₂P 1.0M KOH 14 24.2Herein-Disclosed NiCo₂P_(x) Nanowires 1.0M KOH 58 34.3 ReferenceNi_(1−x)Co_(x)Se₂ nanosheet 1.0M KOH 85 52.0 Reference CoP nanowire/CC1.0M KOH 209 129.0 Reference Co/CoP nanocrystals 1.0M KOH 135 64.0Reference FeP nanowire arrays 1.0M KOH 194 75 Reference MoNi₄/MoO₂cuboids 1.0M KOH 15 30.0 Reference MoP crystals 1.0M KOH ~140 48.0Reference Ni₅P₄ (pellet) 1.0M KOH 48 98.0 Reference Nanoporous Co₂P 1.0MKOH 60 40.0 Reference Bifunctional catalysts η_(10, HER) j_(200, HER)for the HER Electrolytes (mV) (mA cm⁻²) Source FeP/Ni₂P 1.0M KOH 15 346*Herein-Disclosed Porous MoO₂ 1.0M KOH 27 132* ReferenceNi_(0.51)Fe_(0.49)P film 1.0M KOH 82 236* Reference MoS₂/Ni₃S₂ 1.0M KOH110  92* Reference CoP₂/rGO 1.0M KOH 88  84* Reference NiCo₂S₄ nanowirearray 1.0M KOH 210    7.4* Reference Electrodeposited CoP film 1.0M KOH94 480* Reference NiCo₂O₄ 1.0M KOH 110  52* ReferenceEG/Co_(0.85)Se/NiFe-LDH 1.0M KOH 260    4.3* Reference NiFe LDH 1.0M KOH210    8.2* Reference NiFe LDH@DG10 1.0M KOH 66  60* Reference NiFeLDH/Cu NW 1.0M KOH 116 124* Reference NiFeO_(x)/CFP 1.0M KOH 88  62*Reference NiP/Ni 1.0M KOH 130 134* Reference

As seen in FIG. 9b , which provides the Tafel plots corresponding toFIG. 9a , the Tafel slope of the herein-disclosed FeP/Ni₂P catalyst isonly 24.2 mV dec⁻¹, which is lower than that of Ni₂P (117.3 mV dec⁻¹)and Pt (36.8 mV dec⁻¹).

FIG. 9c provides double-layer capacitance measurements for determiningelectrochemically active surface areas of Ni₂P* and FeP/Ni₂P electrodes.FIG. 11a and FIG. 11b provides the scan rate dependence of the currentdensities of Ni₂P/Ni foam and FeP/Ni₂P, respectively, as HER catalystswith scan rates ranging from 1 mV s⁻¹ to 10 mV s⁻¹ at intervals of 1 mVs⁻¹. FIG. 12 provides Nyquist plots of Ni₂P and FeP/Ni₂P for HERmeasured at −1.074 V vs. RHE. To gain further insight into theoutstanding HER activity, the C_(dl) values (FIG. 9c , FIG. 11) wereutilized to compare the active surface area, confirming that both highactive surface area (907.8 mF/cm²) and small charge transfer resistance(FIG. 12) contribute greatly to the outstanding HER catalytic activityof the herein-disclosed FeP/Ni₂P hybrid. It is noted that thecapacitance is different when the same FeP/Ni₂P catalyst was used forHER and OER, which is possibly due to the different origins of activesites for the OER and HER. In particular, we prepared pure Ni₂P*catalyst on Ni foam with similar mass loading in the same growthconditions as FeP/Ni₂P. In this case, the comparative Ni₂P* catalyststill showed catalytic activity inferior to the FeP/Ni₂P hybrid, and hada smaller C_(dl) value (FIG. 9c ). FIG. 13 provides a comparison of thecatalytic HER activity with the same active surface area normalized bythe C_(dl) difference between FeP/Ni₂P hybrid and just Ni₂P catalyst. Asseen in FIG. 13, after normalizing the polarization curves by the activesurface area or Cal difference, the FeP/Ni₂P hybrid still exhibitedbetter catalytic HER activity than pure Ni₂P* catalyst, indicating thatthe herein-disclosed FeP/Ni₂P had better intrinsic activity than pureNi₂P* catalyst.

Calculation of Turn Over Frequency (TOF)

The intrinsic catalytic activity was assessed by the turnover frequency(TOF) for each active site quantified by an electrochemical method.Supposing that every active site was accessible to the electrolyte, theTOF values can be calculated by the following formula:

$\begin{matrix}{{TOF} = {\frac{1}{2}\frac{I}{nF}}} & (1)\end{matrix}$

where these physical variables F, n, and I are corresponding to theFaraday constant (˜96485 C/mol), the number of active sites (mol), andthe current (A) during the LSV measurement in 1 M KOH, respectively. Thefactor ½ is due to fact that two electrons are required to form onehydrogen molecule from two protons. The number of active sites wasdetermined by an electrochemical method. The CV curves were measured in1M PBS electrolyte (pH=7). Due to the difficulty in assigning theobserved peaks to a given redox couple, the number of active sites isnearly proportional to the integrated voltammetric charges (cathodic andanodic) over the CV curves. Supposing a one-electron process for bothreduction and oxidation, we can get the upper limit of the number ofactive sites (n) based on the follow equation:

$\begin{matrix}{n = \frac{Q}{2F}} & (2)\end{matrix}$

where F and Q are the Faraday constant and the whole charge of CV curve,respectively. By this equation and the CV curves, we can obtain thenumber of active sites for the FeP/Ni₂P hybrid is around 3.71×10⁻⁷ mol,while this value is changed to 1.47×10⁻⁷ mol for the pure Ni₂P catalyst,meaning that the FeP/Ni₂P hybrid has active sites 2.5 times that of justNi₂P catalyst. FIG. 14 provides the CV curves recorded on the FeP/Ni₂Phybrid and pure Ni₂P electrodes in the potential ranges between—0.2 Vvs. RHE and 0.6 V vs. RHE in 1 M PBS. The scan rate was 50 mV s⁻¹.

Thus, from the TOF method, the number of active catalytic sites for theFeP/Ni₂P hybrid was found to be around 2.5 times that of the Ni₂P*catalyst, and accordingly the TOF of the FeP/Ni₂P hybrid is calculatedto be 0.163 s⁻¹ at 100 mV overpotential, which is much higher than thatof pure Ni₂P* catalyst (0.006 s⁻¹) at the same overpotential.

FIG. 9d provides a comparison of the overpotentials required at 10 mAcm⁻² between the herein-disclosed FeP/Ni₂P catalyst and availablereported HER catalysts. FIG. 9e provides a comparison of the currentdensities delivered at −200 mV between the herein-disclosed FeP/Ni₂Pcatalyst and available reported HER catalysts. As seen from FIG. 9d andFIG. 9e , the herein-disclosed FeP/Ni₂P hybrid showed outstanding HERactivity compared to other available bifunctional catalysts. To evaluateits stability during electrochemical HER, a long-term cycling test andcontinuous operation for 24 h of hydrogen release at −100 mA/cm² wereperformed in 1 M KOH. FIG. 9f provides polarization curves before andafter the 5000 cycling test, and FIG. 9g provides thechronopotentiometric curve of the herein-disclosed FeP/Ni₂P electrodetested at a constant current density of −100 mA cm⁻² for 24 h. As seenin FIG. 9f and FIG. 9g , the herein-disclosed FeP/Ni₂P electrodedemonstrated good stability

In order to determine the factors contributing to the superior HERactivity of the herein-disclosed FeP/Ni₂P catalyst, density functionaltheory (DFT) calculations were performed on this catalyst. FIGS. 15a-15fprovide the molecular structures of the systems calculated: FIG. 15a—Ni₂P(100); FIG. 15b -FeP(001); FIG. 15c -FeP(010); FIG. 15d -Pt(111)FIG. 15e -FeP(001) on Ni₂P(100), side view; and FIG. 15f -FeP(010) onNi₂P(100), side view. According to FIG. 1d , (021) and (010) latticeplanes are observed on Ni₂P nanoparticles. Since (021) and (010) of Ni₂Phave a simple common perpendicular direction (100), this plane waschosen to model Ni₂P (FIGS. 15a-15f ). Conversely, the two directions,(220) and (103) on FeP, do not share a common simple perpendiculardirection, hence two different directions, (001) and (010), were chosento model FeP. Since the overall system involved two materials, theinteractions between FeP and Ni₂P were modeled by placing FeP on top ofNi₂P, which is reasonable since a Ni foam was used as the material onwhich Ni₂P and FeP were grown. The corresponding lattice distances werechosen to minimize the percent changes in both Ni₂P and FeP. Thehydrogen adsorption energy, ΔG_(H), was calculated by density functionaltheory (DFT) in a GGA level and is shown in Table 3.

TABLE 3 Calculated ΔG_(H) in eV Acidic condition Basic condition ΔG_(H)(eV) (pH = 0) (pH = 14) Ni₂P(100) 0.399 0.306 FeP(001) −0.059 −0.057FeP(010) −0.221 −0.237 FeP(001)/Ni₂P −0.279 −0.255 FeP(010)/Ni₂P −0.238−0.230 Pt −0.184 −0.135

FIG. 9h provides the free energy diagram for ΔG_(H), the hydrogenadsorption free energy at pH=14 on the herein-disclosed FeP/Ni₂Pcatalyst in comparison with Ni₂P and benchmark Pt catalysts. As shown inFIG. 9h , FIGS. 15a-15f , and Table 3, pure Ni₂P (001) leads to arelatively strong exothermic ΔG_(H) (0.306 eV), indicating that it isnot the most active center for the hydrogen evolution electrocatalysis,which was confirmed experimentally (FIG. 9a , FIG. 9b , FIG. 13).However, this hydrogen adsorption energy |ΔG_(H)| is reducedsignificantly to 0.255 and 0.230 eV for a very thin FeP (100) or FeP(010) crystal (˜3 layers), respectively, hybridized atop with Ni₂P.Calculation was confined to a thin layer of FeP crystal, ignoring theparticulate size (5-30 nm), so it was hypothesized that theas-synthesized FeP nanoparticles along with Ni₂P preferentially exposethe most active facets as those of bulk FeP (001) crystal, which resultsin further reduction of |ΔG_(H)| to only 0.06 eV, contributing to thehigh activity not seen in typical FeP crystals. This conclusion is alsosupported by the above experiments regarding the TOF calculation. Thus,both the experiment and theory support that the herein-disclosed hybridcatalyst is an efficient HER electrocatalyst.

Example 4: Overall Water Splitting

Given the outstanding OER and HER activities provided by theherein-disclosed FeP/Ni₂P hybrid electrocatalyst in 1 M KOH electrolyte,the FeP/Ni₂P hybrid was utilized as both anode and cathode in atwo-electrode configuration for overall water splitting in the sameelectrolyte. FIG. 16a provides the polarization curve of FeP/Ni₂P andIrO₂—Pt coupled catalysts in a two-electrode configuration; FIG. 16bprovides an enlarged version at low current density region of FIG. 16a .FIG. 16c provides a comparison of the cell voltages to achieve 10 mAcm⁻² among different water alkaline electrolyzers. FIG. 17 provides acyclic voltammetry (CV) curve (raw data) and corresponding averageactivity calculated from the CV curve of FeP/Ni₂P as a bifunctionalcatalyst for overall water splitting obtained at a scan rate: 1 mV s⁻¹.As seen in FIG. 16a and FIG. 16b , the cell voltage to afford a currentdensity of 10 mA/cm² was as low as 1.42 V, substantially lower than thatof the coupled benchmark IrO₂—Pt catalysts (1.57 V), and superior tomost previously reported bifunctional electrocatalysts, which generallyneed cell voltages higher than 1.50 V to deliver the same currentdensity, as seen in FIG. 16c , Table 4 below, and FIG. 17). This cellvoltage also manifests that the electrical-to-fuel efficiency ofwater-splitting electrolyzers at 10 mA/cm² is dramatically elevated to86.6% using solely this material, potentially positioning it with greatpotential for scale-up water electrolysis with high efficiency and lowcost.

Table 4 provides a comparison of the HER, OER and overall watersplitting activities with available robust bifunctional catalysts. InTable 4, η_(10,HER), η_(10,OER), η_(100,overall), η_(100,overall) andη_(1,7,overall) correspond to the overpotentials of HER, OER catalyzedat 10 mA cm⁻², the cell voltages at 10 and 100 mA cm⁻², and currentdensity at 1.7 V for the overall water splitting, respectively.

TABLE 4 Comparison of the HER, OER and Overall Water SplittingActivities with Available Robust Bifunctional Catalysts. n₁₀, _(HER)n₁₀, _(OER) n₁₀, _(overall) n₁₀₀, _(overall) j_(1.7), _(overall)Catalyst Electrolytes (mV) (mV) (V) (V) (mA cm⁻²) Source FeP/Ni₂P 1.0MKOH 14 154 1.42 1.602   406 Herein- Disclosed Porous MoO₂ 1.0M KOH 27260 1.52 1.8*    67* Reference Ni_(0.51)Fe_(0.49)P film 1.0M KOH 82 2391.53 1.71*    87* Reference MoS₂/Ni₃S₂ 1.0M KOH 110 218 1.56 1.71*    91.4* Reference CoP₂/rGO 1.0M KOH 88 300 1.56 1.912*   31* ReferenceNiCo₂S₄ nanowire array 1.0M KOH 210 260 1.63 2.097*   16* ReferenceElectrodeposited Co-P film 1.0M KOH 94 345  1.64* 1.745*   42* ReferenceNiCo₂O₄ 1.0M NaOH 110 290 1.65 1.842*   16* ReferenceEG/Co0.85Se/NiFe-LDH 1.0M KOH 260 >250* 1.67 1.907*    16.6* ReferenceNiFe LDH 1.0M NaOH 210 240 1.7  2.241*  10 Reference NiFe LDH@DG10 1.0MKOH 66 201  1.44* 1.87*    60* Reference NiFe LDH/Cu NW 1.0M KOH 116 1991.54 1.69*   111* Reference NiFeO_(x)/CFP 1.0M KOH 88 230 1.51 1.73*   70* Reference NiP/Ni 1.0M KOH 130 247 1.61 2.102*   24* Reference*Calculated according to the curves given in the literature.

Although the best bifunctional NiFe LDH catalyst reported recently candeliver 20 mA/cm² at a cell voltage of 1.50 V, which is close to that ofthe herein-disclosed FeP/Ni₂P catalyst (1.48 V), a much larger cellvoltage of 1.70 V is needed to achieve only 60 mA/cm², meaning lowenergy conversion efficiency at high current density. FIG. 16d providesa comparison of the cell voltages to achieve 100 mA cm⁻² among differentwater alkaline electrolyzers. FIG. 16e provides a comparison of thecurrent densities at 1.7 V for the herein-disclosed FeP/Ni₂P catalystwith available non-noble bifunctional catalysts. As seen in FIG. 16d ,nearly all conventional bifunctional electrocatalysts require a cellvoltage greater than 1.69 V to reach 100 mA/cm² for the overall watersplitting. As seen in FIG. 16e , even at 1.7 V cell voltage, most of theknown electrolyzers can only deliver current densities below 110 mA/cm².In contrast, the herein-disclosed FeP/Ni₂P hybrid catalyst can readilydrive water electrolysis at high current densities of 100, 500, and 1000mA/cm² at very low cell voltages of 1.60, 1.72, and 1.78 V,respectively, showing that the herein-disclosed FeP/Ni₂P hybridelectrocatalyst performs outstandingly over the full range of currentdensity. FIG. 16f provides the catalytic stability of the FeP/Ni₂Pcatalysts at 30, 100, and 500 mA cm⁻² for around 40 h. As seen in FIG.16f , the long-term stability of the herein-disclosed FeP/Ni₂P electrodewas tested at 30 and 100 mA/cm² for 36 h, showing no detectable voltagedecay. Moreover, extremely high-current operation of theherein-disclosed electrolyzer at 1.72 V was examined for overall watersplitting at 500 mA/cm², which is a big step toward real industrialapplications. In comparison, commercial alkaline water electrolysisrequires 1.8-2.4 V to generate 200-400 mA/cm², with no previously-knownbifunctional catalysts showing catalytic activities superior to thecommercial ones with good durability at high current density above 200mA/cm². In contrast, as seen in FIG. 16f , the herein-disclosed alkalineelectrolyzer required only 1.72 V to afford 500 mA/cm² while alsoexhibiting excellent stability for more than 40 h confirmed by steadychronopotentiometric testing.

Example 4a: Measurements of Gas Products from Overall Water Splitting byGas Chromatography (GC)

FIG. 18a shows GC signals for the FeP/Ni₂P-based water alkalineelectrolyzer after 20 and 40 min of overall water splitting. FIG. 18bshows the amounts of H₂ and O₂ gases versus time at a constant currentdensity of 100 mA cm². Specifically, as seen in FIG. 18a and FIG. 18b ,using the gas chromatography-based technique, it was discovered that H₂and O₂ are the only gas products during water electrolysis, and themolar ratio between H₂ and O₂ is close to stoichiometric ratio of 2:1,suggesting that nearly all the electrons are actively involved in thecatalytic reaction. This demonstrates outstandingoverall-water-splitting activity of the herein-disclosed FeP/Ni₂P hybridcatalyst, offering great potential for industrial use.

While various embodiments have been shown and described, modificationsthereof can be made by one skilled in the art without departing from thespirit and teachings of the disclosure. The embodiments described hereinare exemplary only, and are not intended to be limiting. Many variationsand modifications of the subject matter disclosed herein are possibleand are within the scope of the disclosure. Where numerical ranges orlimitations are expressly stated, such express ranges or limitationsshould be understood to include iterative ranges or limitations of likemagnitude falling within the expressly stated ranges or limitations(e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numericalrange with a lower limit, R_(L) and an upper limit, R_(U) is disclosed,any number falling within the range is specifically disclosed. Inparticular, the following numbers within the range are specificallydisclosed: R=R_(L)+k*(R_(U)−R_(L)), wherein k is a variable ranging from1 percent to 100 percent with a 1 percent increment, i.e., k is 1percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . 50 percent,51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98percent, 99 percent, or 100 percent. Moreover, any numerical rangedefined by two R numbers as defined in the above is also specificallydisclosed. Use of the term “optionally” with respect to any element of aclaim is intended to mean that the subject element is required, oralternatively, is not required. Both alternatives are intended to bewithin the scope of the claim. Use of broader terms such as comprises,includes, having, etc. should be understood to provide support fornarrower terms such as consisting of, consisting essentially of,comprised substantially of, etc.

Accordingly, the scope of protection is not limited by the descriptionset out above but is only limited by the claims which follow, that scopeincluding all equivalents of the subject matter of the claims. Each andevery claim is incorporated into the specification as an embodiment ofthe present disclosure. Thus, the claims are a further description andare an addition to the embodiments of the present disclosure. Thediscussion of a reference is not an admission that it is prior art tothe present disclosure, especially any reference that may have apublication date after the priority date of this application. Thedisclosures of all patents, patent applications, and publications citedherein are hereby incorporated by reference, to the extent that theyprovide exemplary, procedural, or other details supplementary to thoseset forth herein.

ADDITIONAL DESCRIPTION

The particular embodiments disclosed above are illustrative only, as thepresent disclosure may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered ormodified and all such variations are considered within the scope andspirit of the present disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Whilecompositions and methods are described in broader terms of “having”,“comprising,” “containing,” or “including” various components or steps,the compositions and methods can also “consist essentially of” or“consist of” the various components and steps. Use of the term“optionally” with respect to any element of a claim means that theelement is required, or alternatively, the element is not required, bothalternatives being within the scope of the claim.

Numbers and ranges disclosed above may vary by some amount. Whenever anumerical range with a lower limit and an upper limit is disclosed, anynumber and any included range falling within the range are specificallydisclosed. In particular, every range of values (of the form, “fromabout a to about b,” or, equivalently, “from approximately a to b,” or,equivalently, “from approximately a-b”) disclosed herein is to beunderstood to set forth every number and range encompassed within thebroader range of values. Also, the terms in the claims have their plain,ordinary meaning unless otherwise explicitly and clearly defined by thepatentee. Moreover, the indefinite articles “a” or “an”, as used in theclaims, are defined herein to mean one or more than one of the elementthat it introduces. If there is any conflict in the usages of a word orterm in this specification and one or more patent or other documents,the definitions that are consistent with this specification should beadopted.

Embodiments disclosed herein include:

A: A method of manufacturing a bifunctional electrocatalyst for overallwater splitting comprising oxygen evolution reaction (OER) and hydrogenevolution reaction (HER), the method comprising: growing electrocatalystcomprising primarily metallic phosphides on a three-dimensionalsubstrate by: immersing the substrate in an iron nitrate solution toform a once disposed substrate; subjecting the once disposed substrateto thermal phosphidation with phosphorus powder under inert gas to growmetal phosphides thereupon and form a once subjected substrate; coolingthe once subjected substrate to form a cooled, once subjected substrate;immersing the cooled, once subjected substrate in an iron nitratesolution to form a twice disposed substrate; and subjecting the twicedisposed substrate to thermal phosphidation with phosphorus powder underinert gas to provide an electrode comprising the bifunctionalelectrocatalyst on the three-dimensional substrate.

B: An electrode for overall water splitting, the electrode comprising: asubstrate; and a bifunctional electrocatalyst comprising primarilymetallic phosphides on a surface of the substrate.

C: A method of electrocatalytic water splitting, the method comprising:providing an anode and a cathode, wherein each of the anode and thecathode comprises a uniform distribution of a bifunctionalelectrocatalyst comprising metallic phosphides on a conductivesubstrate; and utilizing the anode and the cathode for alkaline waterelectrolysis, wherein the bifunctional electrocatalyst promotes hydrogenevolution reaction (HER) at the cathode, and oxygen evolution reaction(OER) at the anode.

Each of embodiments A, B and C may have one or more of the followingadditional elements: Element 1: wherein the metallic phosphides compriseprimarily a majority of iron phosphide (FeP) and a minority of dinickelphosphide (Ni₂P). Element 2: wherein the metallic phosphides less thanor equal to about 12.5% weight percent dinickel phosphide (Ni₂P) andgreater than or equal to about 87.5% weight percent iron phosphide(FeP). Element 3: wherein thermal phosphidation is effected at atemperature in the range of from about 350° C. to about 550° C. Element4: wherein subjecting the once disposed substrate to thermalphosphidation, subjecting the twice disposed substrate to thermalphosphidation, or both comprises a thermal phosphidation for a durationof time in the range of from about 0.5 hour to about 1 hours. Element 5:wherein the three-dimensional substrate comprises nickel (Ni) foam,wherein the metallic phosphides of the bifunctional electrocatalystcomprise FeP and Ni₂P, and wherein the electrode comprises or consistsessentially of an FeP/Ni₂P/Ni foam. Element 6: wherein thethree-dimensional substrate comprises one or more of a metallic foam ora carbon cloth paper. Element 7: wherein the metallic foam comprisesnickel (Ni), copper (Cu), iron (Fe), cobalt (Co), titanium (Ti), or acombination thereof. Element 8: wherein the inert gas comprises argon,and wherein the phosphorus comprises red phosphorus. Element 9: whereinsubjecting to thermal phosphidation comprises direct thermalphosphidation in a tube furnace or a chemical vapor deposition (CVD)system or molecular organic chemical vapor deposition (MOCVD) systemunder argon atmosphere. Element 10: wherein the electrocatalyst has ahigh porosity, as evidenced by a porosity of the metallic phosphides onthe substrate that is greater than or equal to a porosity of thesubstrate. Element 11: wherein the electrocatalyst is operable foralkaline water electrolysis. Element 12: wherein the bifunctionalelectrocatalyst exhibits good performance for both the HER and the OER,and is stable at current densities of up to at least 100 mA/cm².

Element 13: wherein the substrate comprises a three dimensionalsubstrate. Element 14: wherein the metal foam comprises nickel (Ni),copper (Cu), iron (Fe), cobalt (Co), titanium (Ti), or a combinationthereof. Element 15: wherein the three dimensional substrate comprisesnickel (Ni) foam, wherein the metallic phosphides comprise primarily acombination of iron phosphide (FeP) and dinickel phosphide (Ni₂P), andwherein the electrode thus comprises or consists essentially of FeP andNi₂P on Ni foam. Element 16: wherein the metallic phosphides compriseprimarily iron phosphide (FeP) and dinickel phosphide (Ni₂P). Element17: wherein the metallic phosphides comprise a majority of ironphosphide (FeP) and a minority of dinickel phosphide (Ni₂P). Element 18:wherein a loading of the bifunctional electrocatalyst comprisingprimarily metallic phosphides is in the range of from about 8 to about13.5 mg/cm². Element 19: wherein a loading of dinickel phosphide (Ni₂P)is in the range of from about 1 to about 2 mg/cm²; wherein a loading ofiron phosphide (FeP) is in the range of from about 7 to about 13 mg/cm²,or a combination thereof. Element 20: wherein the bifunctionalelectrocatalyst has a high porosity, as evidenced by a porosity of theFeP/Ni₂P on the substrate that is greater than or equal to a porosity ofthe substrate.

Element 21: wherein the bifunctional catalyst of the anode has the samecomposition as the bifunctional catalyst of the cathode. Element 22:wherein a loading of the metallic phosphides on the conductive substrateis in the range of from about 8 to about 15 mg/cm². Element 23: whereina loading of dinickel phosphide (Ni₂P) on the conductive substrate is inthe range of from about 1 to about 2 mg/cm²; wherein a loading of ironphosphide (FeP) on the conductive substrate is in the range of fromabout 7 to about 13 mg/cm²; or a combination thereof. Element 24:wherein the conductive substrate comprises nickel foam, and wherein theanode and the cathode thus comprise an FeP/Ni₂P/Ni foam. Element 25:wherein, when operated in 1M alkaline solution, the bifunctionalelectrocatalyst requires a low overpotential of less than 15 mV and 155mV, respectively, to deliver a current density of 10 mA/cm² for the HERand OER, leading to an overall water-splitting activity at 10 mA/cm²with less than 1.5 V. Element 26: wherein the bifunctionalelectrocatalyst has a high porosity, as evidenced by a porosity of thebifunctional electrocatalyst on the conductive substrate that is greaterthan a porosity of the conductive substrate. Element 27: wherein, whenoperated in 1M alkaline solution, the bifunctional electrocatalystyields a current density of at least 100 mA/cm² at an overpotential ofless than or equal to about 225 mV for the OER, exhibits durability forat least 5,000 cycles, is operable for at least 20 hours at 100 mA/cm²,or a combination thereof.

While preferred embodiments of the invention have been shown anddescribed, modifications thereof can be made by one skilled in the artwithout departing from the teachings of this disclosure. The embodimentsdescribed herein are exemplary only, and are not intended to belimiting. Many variations and modifications of the invention disclosedherein are possible and are within the scope of the invention.

Numerous other modifications, equivalents, and alternatives, will becomeapparent to those skilled in the art once the above disclosure is fullyappreciated. It is intended that the following claims be interpreted toembrace all such modifications, equivalents, and alternatives whereapplicable. Accordingly, the scope of protection is not limited by thedescription set out above but is only limited by the claims whichfollow, that scope including all equivalents of the subject matter ofthe claims. Each and every claim is incorporated into the specificationas an embodiment of the present invention. Thus, the claims are afurther description and are an addition to the detailed description ofthe present invention. The disclosures of all patents, patentapplications, and publications cited herein are hereby incorporated byreference. Unless expressly stated otherwise, the steps in a methodclaim may be performed in any order and with any suitable combination ofmaterials and processing conditions.

What is claimed is:
 1. An electrode for overall water splitting, theelectrode comprising: a substrate; and a bifunctional electrocatalystcomprising primarily metallic phosphides disposed on a surface of thesubstrate.
 2. The electrode of claim 1, wherein the substrate comprisesa three dimensional substrate.
 3. The electrode of claim 1, wherein thethree dimensional substrate comprises a metal foam or carbon clothpaper.
 4. The electrode of claim 3, wherein the metal foam comprisesnickel (Ni), copper (Cu), iron (Fe), cobalt (Co), titanium (Ti), or acombination thereof.
 5. The electrode of claim 4, wherein the threedimensional substrate comprises nickel (Ni) foam, wherein the metallicphosphides comprise primarily a combination of iron phosphide (FeP) anddinickel phosphide (Ni₂P), and wherein the electrode comprises orconsists essentially of FeP and Ni₂P on Ni foam.
 6. The electrode ofclaim 1, wherein the metallic phosphides comprise primarily ironphosphide (FeP) and dinickel phosphide (Ni₂P).
 7. The electrode of claim6, wherein the metallic phosphides comprise a majority of iron phosphide(FeP) and a minority of dinickel phosphide (Ni₂P).
 8. The electrode ofclaim 6, wherein a loading of the bifunctional electrocatalystcomprising primarily metallic phosphides is in the range of from 8 to13.5 mg/cm².
 9. The electrode of claim 8, wherein a loading of dinickelphosphide (Ni₂P) is in the range of from 1 to about 2 mg/cm²; wherein aloading of iron phosphide (FeP) is in the range of from 7 to 13 mg/cm²,or a combination thereof.
 10. The electrode of claim 6, a porosity ofthe FeP/Ni₂P on the substrate that is greater than or equal to aporosity of the substrate.
 11. A method of electrocatalytic watersplitting, the method comprising: providing an anode and a cathode,wherein each of the anode and the cathode comprises a uniformdistribution of a bifunctional electrocatalyst comprising metallicphosphides on a conductive substrate; and utilizing the anode and thecathode for alkaline water electrolysis, wherein the bifunctionalelectrocatalyst promotes hydrogen evolution reaction (HER) at thecathode, and oxygen evolution reaction (OER) at the anode.
 12. Themethod of claim 11, wherein the bifunctional catalyst of the anode hasthe same composition as the bifunctional catalyst of the cathode. 13.The method of claim 12, wherein the metallic phosphides compriseprimarily iron phosphide (FeP) and dinickel phosphide (Ni₂P).
 14. Themethod of claim 13, wherein the metallic phosphides comprise a majorityof iron phosphide (FeP) and a minority of dinickel phosphide (Ni₂P). 15.The method of claim 13, wherein a loading of the metallic phosphides onthe conductive substrate is in the range of from 8 to 15 mg/cm².
 16. Themethod of claim 13, wherein a loading of dinickel phosphide (Ni₂P) onthe conductive substrate is in the range of from 1 to 2 mg/cm², whereina loading of iron phosphide (FeP) on the conductive substrate is in therange of from 7 to 13 mg/cm², or a combination thereof.
 17. The methodof claim 16, wherein the conductive substrate comprises nickel foam, andwherein the anode and the cathode comprise an FeP/Ni₂P/Ni foam.
 18. Themethod of claim 17, wherein, when operated in 1M alkaline solution, thebifunctional electrocatalyst requires a low overpotential of less than15 mV and 155 mV, respectively, to deliver a current density of 10mA/cm² for the HER and OER, leading to an overall water-splittingactivity at 10 mA/cm² with less than 1.5 V.
 19. The method of claim 18,wherein a porosity of the bifunctional electrocatalyst on the conductivesubstrate that is greater than a porosity of the conductive substrate.20. The method of claim 18, wherein, when operated in 1M alkalinesolution, the bifunctional electrocatalyst yields a current density ofat least 100 mA/cm² at an overpotential of less than or equal to about225 mV for the OER, exhibits durability for at least 5,000 cycles, isoperable for at least 20 hours at 100 mA/cm², or a combination thereof.